1. Technical Field of the Invention
The present invention relates to a circuit board, a mounting structure for a semiconductor device with bumps, an electro-optic device and an electronic device. More particularly, the present invention concerns a circuit board in which the mounting position is not displaced even when a semiconductor device with fine bumps such as a ball grid array (hereinafter, referred to as a BGA) is mounted by reflowing, a mounting structure for the semiconductor device with the bumps, an electro-optic device and an electronic device including such a circuit board.
2. Related Art
A method of mounting a quad flat package (QFP) on a printed circuit board (PCB) has been widely performed as a mounting method adapted to the tendency of fine-pitch and multiple-pin lead terminals of a semiconductor. This is used in a state in which the QFP that is a flat package having a large number of gull-wing lead terminals on four sides of the package is mounted to a PCB that is normally made of resin or the like and is electrically connected to the conductor of the PCB.
However, problems have been pointed out in that the QFP tends to have low connection reliability because of a short circuit due to a solder bridge during mounting and a shortage in solder as a consequence to the fine-pitch and multiple-pin terminals. The QFP has also the problem of increasing a mounting area on the PCB by an amount of the outward projection of the lead terminals from the package.
Therefore, a mounting method that uses the BGA or a chip-size package (hereinafter, referred to as a CSP) and printed boards on which they are mounted are proposed in order to cope with the tendency to the fine-pitch and multiple-pin semiconductor devices.
Specifically, FIG. 24 shows a printed board in which lands 382 and 384 of a semiconductor package each having a different shape are disposed on the same printed board and the lands to which terminals having the same function in each package are connected with a pattern wire 388. With such a printed board, even when the shape of the package to be mounted to the printed circuit board is changed, it can be mounted on the same printed board without the need to make another printed board.
There is also a method for mounting an electronic part such as a BGA that is difficult to replace and repair so as not to cause poor soldering. More specifically, FIG. 25 shows the schematic flowchart of a mounting method 290 including step 291 of printing solder cream in a predetermined position on the PCB; step 292 of mounting a semiconductor device, by a mounter, on a predetermined position on the PCB on which the solder cream is printed, the semiconductor device having bumps such as a BGA (including a chip size package (CSP) with a pitch of 0.8 mm or less); step 293 of sorting nondefectives and defectives of the semiconductor devices by an X-ray examination; and step 294 of mounting by reflow heating only nondefectives in the semiconductor devices which have passed the X-ray examination.
As FIGS. 26(a) to 26(c) show, a mounting method has also been proposed by which a semiconductor device 346 with bumps and pads 341 of a substrate 343 are bonded by thermocompression via an anisotropic conductive film (hereinafter, referred to as an ACF) 349.
The mounting method using the ACF 349 offers the advantage that the occurrence of a short circuit between adjacent bumps can be efficiently prevented even when the pitch of the bumps 347 is as small as about 0.1 to 0.5 mm as in the CSP and also the many bumps 347 can be electrically connected by one operation.
However, with the conventional printed board, the solder material serving as a conductive material must be accurately printed on a fine land (pad) when a semiconductor device with fine bumps such as a BGA is mounted, thus posing the problems that it takes much time for positioning for printing and printing itself and the position of the printed solder material and the pad are easily displaced. Particularly, since the CSP is pitched more finely than the BGA, it is difficult in practice to accurately print and mount the solder material on the land (pad) of an easily deformable printed board such as a flexible printed circuit (hereinafter, referred to as a FPC).
According to the mounting method shown in FIG. 25, an X-ray examination must be performed before reflow heating, thus disadvantageously increasing the number of process steps, complicating production control, and increasing manufacturing time. The solder cream must be accurately printed on a fine pad, thus posing the problem of taking a lot of time for positioning for printing and printing itself.
The mounting method using the ACF offers the disadvantages that it costs a lot for the ACF and it is difficult to mount it simultaneously with other devices. In other words, the ACF that is mounted by thermo compression bonding and other devices mounted by reflow soldering must be separately mounted in consideration of the order of different mounting processes.
Thus, after due consideration to the above problems, the inventors have found that providing a region where the pitch in the longitudinal direction and the lateral direction of the pads are different and the wire of the pad is preferentially drawn from a wider pitch side of the longitudinal direction or the lateral direction of the pads decreases imperfect application of solder material, thus allowing a semiconductor device with a fine bump to be finely mounted by reflowing even on a relatively easily deformable substrate such as an FPC.
Accordingly, it is an object of the present invention to provide a circuit board, a mounting structure for a semiconductor device with bumps, an electro-optic device and an electronic device in which the mounting position of the semiconductor device is not displaced even when a semiconductor device with fine bumps such as a BGA is mounted by reflowing.